1. Field of the Invention
The present invention relates to a sports racket, such as a tennis, racquetball, squash or badminton racket comprising a frame of a structural laminate having desirable vibration damping and stiffness properties.
2. Related Background Art
The ability to dissipate vibrational energy is an important characteristic of sports rackets. Vibrational modes are present in both the racket frame and the strings, and there have been numerous approaches to damping these vibrational modes. U.S. Pat. No. 4,983,242 to Reed describes a tennis racket made from a tubular member having a layer of viscoelastic material sandwiched between inner and outer layers of fiber-reinforced resin. U.S. Pat. No. 5,374,057 to Lai et al. describes a racket that is made from either a solid or a tubular frame, and which has a damping element secured to either the inside or outside surface of the frame. Neither of these references contains any disclosure of the use of curved fibers, or of the importance of fiber orientation in controlling stiffness and damping properties of structural laminates.
General applications of viscoelastic materials to attenuate noise and vibration in passive damping devices are well known, as disclosed for example in U.S. Pat. No. 3,079,277. Sandwiching elastomeric layers between non-elastomeric layers to produce a vibration-damping laminate is also known, as disclosed in U.S. Pat. No. 4,278,726. The use of opposing fiber orientations to obtain damping in combination with a spring is disclosed in U.S. Pat. No. 3,892,398.
In all of these devices, noise and vibrational energy is damped through development of internal shear stresses in the viscoelastic material. The damping efficiency increases with the amount of internal shear developed. With the exception of U.S. Pat. No. 3,892,398, in which a spring provides energy absorption, damping is produced through tension-tension coupling between the elastomeric and the non-elastomeric layers, resulting in shear development in the viscoelastic material. However, in this type of coupling, shear is developed only at or near the free edges of the viscoelastic material. As a result, after the length of the device reaches the shear length, damping performance is not improved by further increases in length alone.
The tension-shear coupling characteristics of certain fiber-reinforced materials provide another means of creating structural damping. U.S. Pat. No. 5,203,435 to Dolgin describes a composite damping strut having a viscoelastic material sandwiched between two layers of fiber-reinforced composite material, with the fiber-reinforced layers having opposing orientations. The disclosure of Dolgin claims that the tension-shear coupling for layers of linear fibers at an angle to the load direction can produce tension-twist coupling in a tube, thereby producing shear throughout the viscoelastic layer. In fact, this configuration can produce shear only at and near the edges of the viscoelastic layer. Use of a very low-stiffness viscoelastic material can extend the length of the region in which damping occurs, but at the penalty of a reduction of stiffness of the device. The limitations of this edge effect in composite laminates are well known; see, e.g., R. M. Jones, Mechanics of Composite Materials, Hemisphere Publishing (1975). Moreover, when the strut is clamped at the ends, as would be typical in most applications, the non-elastomeric layers will not displace relative to each other. In this case, shear at the edges is eliminated, drastically reducing the desired vibration-damping characteristics. Dolgin also describes the use of xe2x80x9cV-shapedxe2x80x9d plies having opposite orientations in the reinforced layers, and claims that this configuration produces shear throughout the viscoelastic layer. In this case, shear will be produced only at and near the edges of the viscoelastic layer and in the vicinity of the vertex of the xe2x80x9cVxe2x80x9d shape. Finally, Dolgin discloses without explanation or supporting data, in FIG. 8, sine-wave-shaped plies in which the sine waves in the two reinforced layers appear to be 180xc2x0 out of phase.
The present invention is directed to a sports racket comprising a frame formed of an energy absorbing structural laminate. The structural laminate comprises: (a) a first layer of composite material comprising x1 plies; wherein x1 is from 1 to 50; and wherein at least 0.5x1 plies contain fibers arranged in a shape of a continuous curve; wherein said continuous curve is the same or different in different plies; (b) a second layer of composite material comprising x2 plies; wherein x2 is from 1 to 50; and wherein at least 0.5x2 plies contain fibers arranged in a shape of a continuous curve; wherein said continuous curve is the same or different in different plies; and (c) a viscoelastic layer disposed between the first and second layers such that shear strains are substantially distributed throughout the viscoelastic layer when a load is applied to the structural laminate;
provided that no more than 0.75x1 plies of said first layer and no more than 0.75x1 plies of said second layer contain fibers arranged in a shape of sine curves in which: (i) angular axes in said 0.75x1 plies and said 0.75x2 plies are substantially parallel to a principal maximum load direction, and wavelengths and amplitudes are substantially equal; (ii) sine curves within said 0.75x1 plies are substantially in phase, and sine curves within said 0.75x2 plies are substantially in phase; and (iii) a phase difference between said 0.75x1 plies and said 0.75x2 plies is within about 30xc2x0 of 180xc2x0.